Messier 81: Bode's Galaxy

Messier 81 is one of the brightest galaxies in the northern sky and, with its neighbor M82, one of the best galaxy pairs for a small telescope. Both sit high in the north, above the bowl of the Big Dipper.

What it is

M81 is a grand-design spiral galaxy about 11.6 million light-years away, one of the nearest large galaxies beyond our Local Group. Its bright disk covers about 21 by 10 arcminutes, roughly 70,000 light-years across at that distance. NASA notes that its central bulge is much larger than the Milky Way's and that its central black hole has about 70 million times the Sun's mass, roughly 15 times the mass of the one at the center of our galaxy.

M81 is the largest member of the M81 group of galaxies. Tens of millions of years ago it passed close to M82 and NGC 3077. That encounter distorted M82, set off its burst of star formation and also sharpened M81's own spiral pattern. In 1993 a supernova, SN 1993J, appeared in M81 and reached about magnitude 10.5.

When to see it

At +69° declination, M81 never sets from 40°N; it circles the pole all night, all year. It is highest and best placed on spring evenings, crossing the meridian above the pole around 10 p.m. in mid-March and around 8 p.m. in mid-April, when it stands about 61° up in the north. In autumn it is still visible, but low in the north. From the Southern Hemisphere it is visible only from the tropics, low on the northern horizon.

How to find it

Use the Big Dipper's bowl. Start at Phecda, the bottom inner corner of the bowl, and draw a line to Dubhe, the top outer corner, about 10° away. Continue the line the same distance beyond Dubhe. M81 and M82 lie right there. In binoculars or a finder, look for two small glows about half a degree apart: M81 is the brighter, rounder one to the south.

What you will see

Under a dark sky, 10×50 binoculars show M81 as a clear oval glow with a brighter center, with M82 in the same field. A 70–100 mm telescope shows a bright, slightly elongated core inside a larger oval haze, oriented northwest to southeast. In a 200 mm telescope the oval grows to perhaps 15 arcminutes, the core stands out sharply, and on an excellent night you may sense the spiral arms as faint brightenings in the outer glow. It survives suburban skies better than most galaxies, though the outer disk fades away.

History

Johann Elert Bode found M81 and M82 on December 31, 1774, describing a round nebulous patch with a dense center. Pierre Méchain found them again in August 1779, and Charles Messier added both to his catalog after measuring their positions on February 9, 1781. In 1993 a team led by Wendy Freedman used Hubble observations of Cepheid variable stars to measure the distance to M81.